The current total worldwide annual production of hydrogen is over ½ trillion m3 per year. The need for even greater quantities of hydrogen is still a major bottleneck, especially with the new legislative requirements and pressure to produce ultra low sulfur fuels, while available oil resources become heavier with higher contents of sulfur and metals.
The need for additional hydrogen in refineries is clearly growing, currently at the rate of 6.3% per year, and will continue to grow at a rapid pace for the foreseeable future.
In addition, hydrogen-based fuel cells for automotive and stationary applications are gaining popularity for a variety of reasons, including their higher efficiencies and lower emissions. Nonetheless, using pure hydrogen as a fuel in automotive and residential applications faces many obstacles and has many limitations. The infrastructure to deliver hydrogen is inadequate, the refueling of gaseous hydrogen can be slow, and the storage of hydrogen is problematic. The alternatives to producing and using hydrogen range from futuristic solar energy based hydrogen generation to more pragmatic hydrocarbon reforming. Use of liquid/gaseous hydrocarbon fuels to generate hydrogen is being thought of as an immediate solution for large scale hydrogen production. Besides economics and ease of reforming, this option is seen as being more practical than utilizing the existing distribution network.
The conversion of hydrocarbon fuels to hydrogen can be carried out by several processes, including hydrocarbon steam reforming (HSR), partial oxidation reforming (POR), and auto thermal reforming (ATR). Hydrocarbon steam reforming involves the reaction of steam with the fuel in the presence of a catalyst to produce hydrogen and CO as given in equations (1) and (2) for methane, CH4, and isooctane, C8H18 (2,2,4-trimethylpentane), which is used as a surrogate for gasoline. Since steam reforming is endothermic, some of the fuel must be burned and the heat transferred to the reformer via heat exchangers.CH4+H2O≈CO+3H2, ΔH°298=+206.2 kJ/mol  (1)C8H18+8H2O≈8CO+17H2, ΔH°298=+1273.2 kJ/mol  (2)
Partial oxidation involves the reaction of oxygen with fuel to produce hydrogen and CO as illustrated in equations (3) and (4), when the oxygen-to-fuel ratio is less than that required for total combustion, i.e., complete conversion to CO2 and H2O.CH4+½O2≈CO+2H2, ΔH°298=−35.7 kJ/mol  (3)C8H18+4O2≈8CO+9H2, ΔH°298=−158.1 kJ/mol  (4)
Partial oxidation can be conducted with a catalyst (catalytic partial oxidation) or without a catalyst (non-catalytic partial oxidation). The reaction rates are much higher for partial oxidation than for steam reforming, but the hydrogen yield per carbon in the fuel is lower. Non-catalytic partial oxidation requires reaction temperatures above 1000° C. to achieve rapid reaction rates. Although the reaction is exothermic, some of the fuel must be combusted because the amount of heat generated by the reaction is not sufficient to preheat the feed to achieve optimal rates. Recently, there has been interest in catalytic partial oxidation since it operates at lower temperatures than the non-catalytic route. The lower operating temperatures provide better control over the reaction, thus minimizing coke formation and allowing for a wider choice of materials of construction for the reactor.
Catalytic partial oxidation reforming of natural gas is being tested in pilot plants for gas to liquid (GTL) processes. In these cases, one of the advantages is that the syngas having a lower H2/CO molar ratio can be directly used for successive catalytic converters to produce synthetic liquid products. Although the large endothermic heat for the steam-reforming of natural gas is avoided by the exothermic partial oxidation heat, the hydrogen atoms in water, i.e., the source of cheap and plentiful hydrogen, is not utilized as a part of the hydrogen source. Therefore, for the purpose of hydrogen production, this method is not sufficient. Furthermore, this process cannot avoid combustion of the feed gas and the produced gases, resulting in a decrease of selectivity to H2 and/or CO.
Auto thermal reforming involves the reaction of oxygen, steam, and fuel to produce hydrogen and CO2, and can be viewed as a combination of partial oxidation and steam reforming as given in equations (5) and (6). In essence, this process can be viewed as a combination of POR and HSR.CH4+½O2+H2O≈CO2+3H2, ΔH°298=−18.4 kJ/mol  (5)C8H18+4O2+8H2O≈8CO2+17H2, ΔH°298=−236.7 kJ/mol  (6)
The choice of the reaction process to be used for on-board reforming depends on many factors, including the operating characteristics of the application (e.g. varying power demand, rapid startup, and frequent shutdowns) and the type of fuel cell stack. HSR is heat transfer limited and as such does not respond rapidly to changes in the power demand (i.e. “load following”). When power demand rapidly decreases, the catalyst can overheat, causing sintering, which in turn results in a loss of activity. ATR can overcome the load following limitations of HSR, because the heat required for the endothermic reaction is generated within the catalyst bed, a property that allows for more rapid response to changing power demands and faster startup.
In order to supply the large quantity of heat necessary for steam reforming, auto thermal methods involve the a priori combustion of feedstock before entry into the catalytic reformer; the heated gas is then introduced into the catalyst bed. Therefore, the heat supply is limited by the heat capacity of the reactant gases, and does not achieve essential improvements. More recently, the combustion of a part of the hydrocarbon feed has been carried out using catalytic combustion. However, since catalytic combustion is limited by the maximum catalyst-bed temperature of around 1000-1100° C., the situation is not essentially different from the a priori homogeneous combustion.